This invention relates generally to photoimagable compositions, utilized in the electronics industry, and more particularly to a cationically polymerizable epoxy resin system having photoinitiators which resin system has improved heat withstanding properties, improved expansion coefficient, improved rheological properties and also improved photoimaging properties.
There are many different instances where photoimagable compositions are utilized in various industrial processes. In one particular process a photoimagable composition is utilized as a solder mask and protective coating by applying the composition to the underlying printed circuit board. Thereafter photolithographic techniques are employed to reveal various underlying structures on the board while masking others so that solder may be applied by various solder applying processes to the exposed structures. During the solder applying process the solder will adhere to the exposed underlying components and be prevented from adhering where the remaining material operates as a solder mask.
The solder mask may also function as a protective coating which protects the underlying components from external conditions.
A solder mask can be applied by the appropriate methods; for example curtain coating, spray coating, silk screening and dry film. Solder mask material requires certain rheological properties for effective coating. Further, the solder mask must have the properties of providing efficient transmission of the light or other exposing radiation so as to photolyze the photoinitiator through whatever thickness of material is applied. The solder mask must demonstrate good adhesion to the substrate circuit board and the coating must possess a coefficient of thermal expansion similar to the surrounding material, such as the solder. The solder mask should possess good dielectric properties to provide insulation, it should also provide a good resolution, that is, upon development, the unmasked portions to be soldered should be sharply defined. Also, of course, if the material is to be used as a solder mask, the solder mask must possess appropriate physical and chemical properties to withstand the application of the solder material without significant deterioration or degradation and maintain its coverage over the portions of the board wherein solder is to be masked. If it is to be used for other purposes, other properties may be required.
Different types of soldering processes require different temperatures to reflow. However, in the process for joining chips to board by the "C4" (controlled, collapse, chip, connection process) in a C4 furnace, the temperature is typically 360.degree. C. A typical C4 furnace cycle involves going from 300.degree. C. to 360.degree. C., where it remains for about 11/2-2 minutes before returning to 300.degree. C. However, due to the need for rework, the board may be subject to repeat cycles in the furnace.
In the prior art, frequently a patterned chromium layer was utilized as a solder resist mask for chip joining at 360.degree. C. in a C4 furnace. This process involves many steps, therefore, is not cost efficient and also presents a problem with the disposal of processing waste. Another problem is the inability of the chromium layer, which by nature is very thin, to confine the solder conveniently to prevent the flow into unwanted areas.
There is no known solder mask available which is stable at C4 furnace temperatures of 360.degree. C. and which also has acceptable resolution especially for circuits with line height up to 0.002 inches. Thus, a photo patternable solder resist mask demonstrating good adhesion to the substrates, capable of withstanding 360.degree. temperatures, exhibiting good resolution while having a coefficient of expansion compatible with the other layers would be beneficial.
There have been many prior art proposals for different photoimagable compositions including many that use epoxies. Examples of these are found in the following U.S. Pat. Nos.: 4,279,985; 4,548,890; 4,351,708; 4,138,255; 4,069,055; 4,250,053; 4,058,401; 4,659,649; 4,544,623; 4,684,671; 4,624,912; 4,175,963; 4,081,276; 4,693,961; and 4,442,197. All of these patents show various resins and photoinitiators for use in photoimagable compositions many of which are useful as solder masks. However none of them teach or suggest the specific composition of the present invention particularly with a controlled coefficient of thermal expansion and clearly match that of solder.
Another use of a photoimagable dielectric is for building fine line redistribution layers on circuit boards. While there are photosensitive dielectrics, they have a high coefficient of thermal expansion and/or they are not capable of withstanding high temperatures (above 360.degree. C.). This factor, combined with high coefficiency of thermal expansion, leads to cracking and degradation of the redistribution layer.
The present invention, due to its low coefficient of thermal expansion, its ability to withstand high temperatures and its photopatternable characteristics, is useful as a fine line redistribution layer.